The rational development of new antineoplastic agents directed against tubulin, a protein critical for cell division, requires greater understanding of the interactions between the polypeptide subunits of tubulin and its two tightly bound guanine nucleotides. Because tubulin is very unstable initial efforts have been directed at finding stabilizing agents and improving purification methodology. In addition we have been characterizing conditions leading to the dissociation of tubulin into its subunits.